Using large-scale recordings of neuronal spiking activity, self-organized cell assembly sequences, serving mnemonic and planning functions, have recently also been described in the mammalian brain and shown how they move the cognitive content forward or back in time (Pastalkova et al., 2008). Numerous experiments have demonstrated that hippocampal neurons show place-related firing while the rat explores or traverses its environment so that each assembly of hippocampal principal cells defines a particular position of space (O’Keefe and Nadel, 1978; Wilson and McNaughton, 1993). It has been assumed that sequential activity of hippocampal ‘place cell assemblies’ emerges in response to the changing constellation of environmental inputs (O’Keefe and Burgess, 1996) or to body motion-derived cues (McNaughton et al., 1996) that is that they are ‘driven’ by sensory inputs. However, perpetually changing hippocampal assembly sequences could also be observed during the delay part of a memory task in the absence of changing sensory or feedback cues (Fig. 4A). Importantly, several measures of the place cell metric, including the duration of activity episodes of the neurons and the temporal relationship of their spikes relative
